1. Field of the Invention
This invention relates to photo detector cells and more particularly to a photo detector cell in which the photo sensitive portion of the device is contained within a region of one conductivity type substantially enclosed by a second conductivity type material.
2. Description of the Prior Art
Photo detector cells formed in semiconductor bodies are well known to the prior art. U.S. Pat. Nos. 3,529,217, 3,812,581, and 3,836,793 all teach that the socalled parasitic currents in the backbiased isolation junction should be reduced by creating a shielded very, very thick epitaxial layer overlying a very thick subcollector region, buried so deep that minority carriers are generated so far above it that this subcollector acts as a reflector of minority carriers. U.S. Pat. No. 3,529,217 further teaches the epitaxial layer should be coated with an opaque material extending at least one minority diffusion length from the isolation sidewalls to prevent diffusion of minority carriers to these walls.
Thus all of these prior art patents teach that by extending the buried subcollector layer more deeply into the substrate radiation will not penetrate to the subcollector substrate junction and minority charge carriers produced at the radiation sensitive P-N junction will not diffuse down to the subcollector substrate junction and that photons absorbed within the lower half of the buried collection region will not contribute to the photo current.